The broad objective is to gain an understanding of the role of the nucleus in regulating terminal differentiation of skeletal muscle cells growing in vitro. A second objective is to determine to what extent nuclear processes are also involved in maturation, growth and atrophy of differentiated muscle fibers grown under conditions which we have shown to induce either hypertrophy or atrophy. Specifically, we will define optimal conditions for the isolation of muscle nuclei from myogenic cultures using both aqueous and non-aqueous techniques. Following this we will characterize the RNA components of the muscle nucleus during terminal differentiation by formamide polyacrylamide gel electrophoresis and hybridization to cDNA probes made from purified HnRNA. The second phase of this research will be to characterize the structure and components of chromatin isolated from purified nuclei in different stages of terminal differentiation. We will examine the nucleosome structure of chromatin and also the non-histone chromosomal proteins by two-dimensional gel electrophoresis. The last phase of this research will be toward characterizing nuclear functions in vitro. Specifically, we will examine RNA transcription in isolated muscle nuclei to understand how the changes in gene expression observable in myogenic cultures (e.g., synthesis of muscle-specific proteins following cell fusion) are mediated at the primary level of RNA transcription. Finally, we plan to construct a linked transcriptional-translation system from myogenic nuclei and a reticulocyte cell-free protein-synthesizing system in order to determine whether isolated nuclei can transcribe the complete myosin heavy chain mRNA.